Device and methods for color corrected oct imaging endoscope/catheter to achieve high-resolution

ABSTRACT

The present invention is directed to an achromatic endoscope which employs a diffractive microlens. Along with a broadband rotary joint and a custom 800 nm SD-OCT system, ultrahigh-resolution 3D volumetric imaging over a large area becomes possible. The diffractive microlens can be used directly with a GRIN lens, making the endoscope design simpler and cost effective. Preliminary ex vivo 3D intraluminal imaging was performed with the endoscope in conjunction with a home-built broadband rotary joint and a spectral-domain OCT system, demonstrating the performance of the diffractive endoscope. Considering the miniature OCT imaging probe is the required component for using the OCT technology in internal organs, the proposed approach will have a broad impact on endoscopic OCT imaging by improving OCT resolution in any applications that involve a miniature OCT probe, as intravascular OCT imaging, gastrointestinal (GI) tract imaging, airway imaging etc.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/878,885 filed on Sep. 17, 2013 and to U.S.Provisional Patent Application No. 61/948,826 filed on Mar. 6, 2014,both of which are incorporated by reference, herein, in their entirety.

GOVERNMENT SUPPORT

This invention was made with government support under R01EB007636awarded by the National Institutes of Health and the National Instituteof Biomedical Imaging and Bioengineering and R01CA153023 awarded by theNational Institutes of Health and the National Cancer Institute. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to medical imaging. Moreparticularly, the present invention relates to a device and method forcolor corrected OCT imaging.

BACKGROUND OF THE INVENTION

OCT is a noninvasive, high-resolution optical imaging technology capableof real-time imaging of tissue microanatomy with a few millimeterimaging depth and can be envisioned as an optical analog of ultrasoundB-mode imaging, except that it utilizes near infrared light rather soundwaves. Compared to ultrasound, OCT does not require a matching gel andthe resolution of OCT can be 50-100 times finer than ultrasound. OCT canthus function as a form of “optical biopsy”, capable of assessing tissuemicroanatomy and function with a resolution approaching that of standardhistology but without the need for tissue removal. The axial resolutionof OCT is governed by the spectral bandwidth of the light source and itis inversely proportional to the source spectrum bandwidth. Chromaticaberration in the OCT imaging optics will alter the backreflectedspectrum from the target, resulting in the loss of OCT axial resolution.The change in the backreflected spectrum from the sample could alsoresult in the increase in the side lobes of the OCT imaging signal,which again will lead to the loss of OCT axial resolution. In addition,as in conventional imaging optics, the chromatic aberration will focuslight of different colors to different spots, thus degrading the OCTlateral resolution as well. In a benchtop imaging system such as amicroscope, chromatic aberration in the imaging optics is routinelycorrected by using achromatic lenses (e.g. lenses made of multi elementswith different refractive index profiles and surface curvatures). But,such approaches are not cost effective or practical to be implemented inminiature OCT imaging probes.

Miniature endoscopes are a critical component in the OCT technology,enabling translational applications for imaging internal luminal organssuch as the gastrointestinal tract or airways. Most OCT endoscopesdeveloped so far were designed for imaging at 1300 nm, which provides2-3 mm imaging depth and 8-30 μm axial resolution. However, there is anincreasing need to develop an ultrahigh-resolution OCT endoscope forresolving fine structures (e.g. under 5 μm) such as airway smooth muscleor structural changes associated with early stage diseases. Benefitingfrom the availability of broadband light sources at 800 nm,ultrahigh-resolution OCT imaging has been demonstrated at suchwavelength with bench-top systems. For the endoscopic setting, due tothe challenges such as management of chromatic aberration over abroadband spectral bandwidth, there are only few achromatic endoscopicsetups. The designs in those endoscopes are rather complicated andexpensive, involving multi-element achromatic microlenses.

Accordingly, there is a need in the art for a miniature OCT device and acost-effective and practically implemented method for color correctedOCT imaging.

SUMMARY OF THE INVENTION

The foregoing needs are met, to a great extent, by the present inventionwhich provides a device for obtaining OCT images from a subjectincluding a miniature OCT imaging probe configured to obtainhigh-resolution images of the subject and a diffraction elementconfigured to mitigate wavelength dependent aberration in thehigh-resolution images obtained by the OCT imaging probe.

In accordance with an aspect of the present invention, the diffractionelement includes a diffractive lens. The diffractive lens is positionedat a distal end of a compound lens within the OCT imaging probe. Thediffractive lens can have a high diffraction efficiency over a broadspectral range, such as approximately 800 to approximately 1050 nm. Thewavelength dependent aberration takes the form of a chromaticaberration.

In accordance with another aspect of the present invention a method formitigating achromatic aberration in OCT imaging includes using adiffraction element integrated into miniature imaging optics of an OCTimaging probe, wherein the OCT imaging probe comprises a broadbandlightsource. The method also includes reducing a longitudinal focalshift of the broadband light source, such that different colors of lightin the broadband light source will be focused to a small spot forachieving high lateral resolution. Additionally, the method includesminimizing distortion to a backreflected spectral at a given imagingdepth, such that OCT axial resolution is improved to an optimal axialresolution afforded by the broadband lightsource.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings provide visual representations, which will beused to more fully describe the representative embodiments disclosedherein and can be used by those skilled in the art to better understandthem and their inherent advantages. In these drawings, like referencenumerals identify corresponding elements and:

FIG. 1A illustrates a photo of a representation of a miniature OCTimaging probe/endoscope for internal luminal imaging.

FIG. 1B illustrates a photo image of an OCT imaging probe situatedwithin a transparent plastic balloon for imaging of large lumens.

FIG. 1C illustrates a representative OCT image of a human esophagusshowing the difference between normal and cancerous tissue.

FIG. 2 illustrates a schematic diagram of a distal end of a miniatureOCT probe.

FIG. 3 illustrates a schematic diagram of a representative pattern of adiffractive element or mask, according to an embodiment of the presentinvention.

FIGS. 4A-4C illustrate representative schematic diagrams of a distal endof an imaging probe with a built in diffractive element, according to anembodiment of the present invention.

FIG. 5 illustrates a graphical representation of a longitudinal focalshift for a conventional GRIN lens for a conventional (dashed line) anda diffractive (solid line) imaging probe.

FIG. 6 illustrates a graphical view of a spectra backreflected by amirror placed at a focal plane and a plane 3 Rayleigh lengths (3Z_(R))away from the focal plane for a conventional GRIN lens-based OCT imagingprobe catheter and for a diffractive OCT imaging catheter.

FIG. 7A illustrates a schematic diagram of an OCT endoscope according toan embodiment of the present invention.

FIG. 7B illustrates a spectra backreflected by a mirror placed at afocal plane and a plane 3 Rayleigh lengths (3Z_(R)) away from the focalplane for a conventional GRIN lens-based OCT imaging probe catheter andfor a diffractive OCT imaging catheter.

FIG. 7C illustrates a schematic diagram of a configuration of an 800 nmbroadband endoscopic SC-OCT system.

FIG. 7D illustrates a graphical view of fluctuation of a rotary jointcoupling efficiency at 5 fps.

FIGS. 8A-8D illustrate ex vivo real time images of a guinea pigesophagus and a rat trachea.

FIGS. 9A and 9B illustrate schematic diagrams of the imaging probe atits distal end with a built in diffractive element and a micromotor,according to an embodiment of the present invention.

FIGS. 10A and 10B illustrate schematic diagrams of the imaging probe atits distal end with a built-in diffractive element and a beam scanner,according to an embodiment of the present invention.

DETAILED DESCRIPTION

The presently disclosed subject matter now will be described more fullyhereinafter with reference to the accompanying Drawings, in which some,but not all embodiments of the inventions are shown. Like numbers referto like elements throughout. The presently disclosed subject matter maybe embodied in many different forms and should not be construed aslimited to the embodiments set forth herein; rather, these embodimentsare provided so that this disclosure will satisfy applicable legalrequirements. Indeed, many modifications and other embodiments of thepresently disclosed subject matter set forth herein will come to mind toone skilled in the art to which the presently disclosed subject matterpertains having the benefit of the teachings presented in the foregoingdescriptions and the associated Drawings. Therefore, it is to beunderstood that the presently disclosed subject matter is not to belimited to the specific embodiments disclosed and that modifications andother embodiments are intended to be included within the scope of theappended claims.

The present invention is directed to an achromatic endoscope whichemploys a diffractive microlens. Along with a broadband rotary joint anda custom 800 nm SD-OCT system, ultrahigh-resolution 3D volumetricimaging over a large area becomes possible. The diffractive microlenscan be used directly with a GRIN lens, making the endoscope designsimpler and cost effective. Preliminary ex vivo 3D intraluminal imagingwas performed with the endoscope in conjunction with a home-builtbroadband rotary joint and a spectral-domain OCT system, demonstratingthe performance of the diffractive endoscope. Considering the miniatureOCT imaging probe is the required component for using the OCT technologyin internal organs, the proposed approach will have a broad impact onendoscopic OCT imaging by improving OCT resolution in any applicationsthat involve a miniature OCT probe, such as intravascular OCT imaging,gastrointestinal (GI) tract imaging, airway imaging etc.

Miniature OCT imaging probes are an essential component for manytranslational applications of OCT such as intravascular imaging for highrisk plaque detection, intra esophageal or GI tract imaging for cancerdetection and many others. FIGS. 1A-1C illustrate exemplary OCT probesand an exemplary resultant image. Particularly FIG. 1A illustrates aphoto of a representation of a miniature OCT imaging probe/endoscope 10for internal luminal organ imaging. The probe was encased in atransparent plastic tube 12. FIG. 1B illustrates an OCT imaging probe 10situated within a transparent plastic balloon 14 for imaging of largelumens (such as the GI tract). The device illustrated in FIG. 1B canalso include a rotary joint 16 for 3D imaging. FIG. 1C illustrates arepresentative OCT image of a human esophagus showing that the normal(layered structure) and cancerous (nodules) regions can be discerned.

FIG. 2 illustrates a schematic diagram of a distal end optics design fora miniature OCT imaging probe 10, which entails (not limited to) asingle-mode optical fiber (SMF) 18, a glass rod spacer 20, a miniaturebeam focusing lens 22, a micro beam reflector 24, a protective guard 26,and a transparent plastic sheath 12. In miniature OCT imaging probes, aGRIN lens is often used for beam focusing which is cost effective andeasy to implement. However, a GRIN lens comes with chromatic aberration,compromising both the axial and lateral resolution. Correction ofchromatic aberration in such miniature imaging probes becomes verychallenging due to the probe size (diameter and rigid length)restriction. Although the concept of using a multi-element lens similarto a microscope objective can be introduced to the miniature OCT imagingprobes for correcting the chromatic aberration (as we demonstratedbefore), this approach would be very challenging and impractical due toprohibitive cost and increased probe size. Optical glue 28 can be usedto couple the SMF 18 to the spacer 20. FIG. 2 also illustrates the pointof the beam focus 30 after the beam is reflected off of micro-reflector24.

The present invention provides a solution to overcome the long-existingproblem of chromatic aberration in miniature OCT probes by introducing adiffractive element/mask to the imaging lens. The diffractive elementwill diffract light of different wavelengths to slightly differentdirections, which effectively changes the beam path for each wavelength.With a proper design, the path changes induced by the diffractiveelement/mask can be opposite to the changes caused by chromaticaberration, thus compensating the chromatic aberration.

FIG. 3 illustrates a representative schematic of such a diffractiveelement/mask for use with an OCT probe according to the presentinvention. In this design, the longer wavelength can be bent moretowards the optical axis, which is opposite from the chromaticaberration effect. The diffractive element/mask 32 can be made verysmall in size and very large in quantity (through massmicrofabrication), and it can be easily introduced to the imaging opticsof an OCT probe at its distal end, e.g. by sandwiching between someoptics or attached to the end of the GRIN lens. As illustrated in FIG. 3a longer wavelength (λ₂) will be bent more towards the optical axis thanshorter wavelength (λ₁), which is opposite from what occurs in chromaticaberration. Thus, the combination of such a diffractive element with afocus lens can mitigate chromatic aberration.

FIGS. 4A-4C illustrate schematic diagrams of several configurations fora miniature imaging probe with a diffractive element according to anembodiment of the present invention. As illustrated in FIGS. 4A, 4B, and4C, the OCT device 100 includes an SMF 108 coupled to spacer 110 withoptical glue 118. A GRIN lens 112 is disposed at a proximal end of thespacer 110. A micro reflector 114 is positioned proximally of the GRINlens 112 and directs the beam toward beam focus 120. The OCT device 100is disposed within a plastic tube 102 and also includes protective guard116. Each of FIGS. 4A, 4B, and 4C also include a diffractive element 122incorporated into the design. As illustrated in FIG. 4A, the diffractiveelement 122 is attached to the end of the GRIN lens 112. As illustratedin FIG. 4B, the diffractive element 122 is sandwiched between two piecesof the GRIN lens 112 where the beam is nearly parallel to the opticalaxis, and as illustrated in FIG. 4C, a micro beam splitter 126 and amirror 124 are built in at the distal end to form a common pathdiffractive imaging probe.

Simulations have been performed to investigate the performance of theproposed approach. As shown in FIG. 5, the chromatic aberration of atypical GRIN lens based miniature OCT imaging probe, represented by thelongitudinal focal shift, is about 100 um. When a properly designeddiffractive element is added to the distal end optics of the miniatureimaging probe, the chromatic aberration is dramatically reduced,resulting in a much smaller longitudinal focal shift down to 10 um. FIG.5 illustrates a graphical view of a longitudinal focal shift for aconventional GRIN lens based OCT imaging probe (dashed line) and for adiffractive OCT imaging probe which has a built-in diffractive element(solid line) over the wavelength range of 780-1020 nm. Longitudinalfocal shift represents the severity of chromatic aberration in animaging probe. It is noticed that the longitudinal focal shift isreduced by almost 10 times (i.e. from about 100 μm down to about 10 μm)when using a diffractive element, showing the effective correction ofchromatic correction by the diffractive element.

The proposed approach is demonstrated by implementing an off-the-shelfdiffractive element to a miniature OCT imaging probe. It is noted thatthe diffractive element in the proof-of-the-concept experiments was notoptimized for the specific broadband OCT light source with a spectralbandwidth from 780 nm to 1020 nm. Instead it only covered a portion(880-1120 nm) of our OCT source spectrum. To demonstrate the feasibilityof the proposed concept, the spectra backreflected from a mirror placedat the focal plane of the imaging probe was measured and also measuredat other parallel planes with a given distance away from the focalplane. In an ideal case (i.e. for an imaging probe without any chromaticaberration), the backreflected spectra should not change much with themirror position.

FIG. 6 illustrates a graphical view of the spectra backreflected from amirror placed at the focal plane and a parallel plane 3ZR away from thefocal plane (i.e. 3 times of the Rayleigh length which is about 1.5times of the standard depth of view). The spectra experienced somechanges when the mirror was moved away from the focal plane,particularly at the shorter wavelength side. This change is expectedsince the diffractive element was not specifically designed toaccommodate the whole spectrum of the OCT light source and it was onlysupposed to cover the wavelengths longer than 880 nm. For comparison,the backreflected spectra were also collected by a conventional OCTimaging probe with the same design as the diffractive probe except thatthis conventional probe does not have the diffractive element. Clearlythe backreflected spectrum was severely modified when the mirror wasmoved away from the focal plane, suggesting the basic concept that theproposed diffractive imaging probe works in correcting/mitigatingchromatic aberration. With a specifically designed diffractive elementaccording suitable for the OCT source spectrum, more improvement onchromatic aberration in a miniature imaging probe will be achieved.

FIG. 7A illustrates a schematic diagram of an endoscope according to anembodiment of the present invention. In this OCT device 100 A 125 μmsingle-mode fiber 108 at 800 nm is glued onto a compound lens 110 whichis made of a glass rod and a GRIN rod lens with a 1 mm diameter. Tomanage the chromatic aberration, a custom-made diffractive lens 122 withhigh diffraction efficiency over a broad spectral range (800-1050 nm)but very weak focusing power is placed behind the compound lens 110. A45° reflector 114 is attached to the end of the endoscope to bend thebeam 90° for circumferential imaging. Another endoscope with the samedesign as the above one but without the diffractive lens was also builtfor comparison. To show the effect of the diffractive lens oncompensating chromatic aberration, the spectrum reflected by a mirror atthe focal point of the endoscope is compared with that reflected atthree Rayleigh lengths (i.e. 3ZR) away from the focal point for bothendoscopes, as illustrated in FIG. 7B. The results clearly indicate muchreduced chromatic aberration in the endoscope with the aid ofdiffractive lens. The SD-OCT endoscopic imaging system is illustrated inFIG. 7C, where a broadband super luminescence diode (SLD) from SuperlumLtd. with a full spectral bandwidth of ˜240 nm centering at 870 nm isemployed as the light source. The light source is delivered into thesample and reference arms through a broadband 50/50 fiber coupler. Inthe sample arm, a home-made broadband fiber rotary joint is used toconnect the stationary fiber and the endoscope that is to be rotated.The rotary joint is mounted on a translational stage to enable 3Dvolumetric imaging. The fluctuation of light coupling efficiency of therotary joint was controlled as low as 12% as shown in FIG. 7D. In orderto match the dispersion in two arms, a prism pair was inserted intoreference arm to minimize the air gap in the reference arm. The residualdispersion mismatch between the two arms was numerically compensated.For detection, a custom-designed, home-built linear-in-wavenumberspectrometer is employed. The line scan camera has 2048 pixels and amaximum line scan rate of 70 k/second at 12 bit resolution. Real-timeOCT imaging is rendered by a custom C++ program, which controls thesystem synchronization, real-time data acquisition, signal processing,data storage, etc. The endoscope with the diffractive lens is measuredto have a 3.75 μm axial resolution in air and 6.15 μm lateralresolution.

Real time ex vivo imaging study of guinea pig esophagus was performedusing the diffractive endoscope along with the SD-OCT system. Forcomparison, the same tissue was also imaged with an endoscope without adiffractive lens. FIGS. 8A and 8B illustrate representativecross-sectional esophageal images acquired by the endoscope with andwithout a diffractive lens, respectively. The OCT system was running at5 frames per second (fps) and each frame consisted of 2048 A-scans.Layer structures such as mucosa, submucosa and muscular layer can beidentified on FIG. 8A, while some parts of the muscular layer becomedifficult to resolve on FIG. 8B. It is clear that the quality of the OCTimage acquired with the endoscope without a diffractive lens experienceddramatic degradation. The imaging results indicate that the reducedchromatic aberration by using a diffractive lens is able tosignificantly improve the OCT image quality. FIG. 8C shows arepresentative 3D volumetric image of the esophagus acquired by thediffractive endoscope by pulling back the rotating endoscope with a 10μm pitch. In addition to esophagus, ex vivo rat trachea was also imagedusing the diffractive endoscope running at 10 fps. One representativecross-sectional image is shown in FIG. 8D. Some fine structures such asepithelium, cartilage, glands and smooth muscle can be observed.

FIGS. 9A and 9B illustrate schematic diagrams of the imaging probe atits distal end with a built in diffractive element and a micromotor,according to an embodiment of the present invention. As illustrated inFIGS. 9A and 9B, the OCT device 200 includes an SMF 208 coupled tospacer 210 with optical glue 218. A lens 212 is disposed at a proximalend of the spacer 210. A micro reflector 214 is positioned proximally ofthe lens 212 and directs the beam toward beam focus 220. The OCT device200 is disposed within a plastic tube 202 and also includes protectiveguard 216. Each of FIGS. 9A and 9B also include a diffractive element222 incorporated into the design. More particularly, FIG. 9A illustratesthat the diffractive element 222 is attached to the end of a microlens212. With respect to FIG. 9B, the diffractive element 222 is sandwichedbetween the two microlenses 212, such that the beam is nearly parallelto the optical axis. Circumferential imaging is performed by rotating abeam reflector or microprism. FIG. 9A also illustrates a micromotor 226to rotate the reflector incorporated into the OCT device 200 and also acut-opening or transparent window 224.

FIGS. 10A and 10B illustrate schematic diagrams of the imaging probe atits distal end with a built-in diffractive element and a beam scanner,according to an embodiment of the present invention. As illustrated inFIGS. 10A and 10B, the OCT device 300 includes an SMF 308 coupled tospacer 310 with optical glue 318. A lens 312 is disposed at a proximalend of the spacer 310. A micro reflector 314 is positioned proximally ofthe lens 312 and directs the beam toward beam focus 320. The OCT device300 is disposed within a plastic tube 302 and also includes protectiveguard 316. Each of FIGS. 10A and 10B also include a diffractive element322 incorporated into the design. More particularly, FIG. 10Aillustrates a PZT actuated fiber scanner 324 and FIG. 10B illustrates aMEMS beam scanner 328. Beam scanning is performed at the distal end ofthe probe by using a micromotor to rotate a reflector (for achievingcircumferential imaging), or a fiber scanner 324 or an MEMS scanner 328to scan the beam (where forward-viewing imaging is possible). Thus theentire endoscope does not have to rate for achieving beam scanning Thisavoids the use of a fiber-optic rotary joint, which could be thebottleneck for sped or spectral bandwidth, in addition to all thepotential artifacts associated with fiber endoscope rotation. Asillustrated in FIG. 10A a SMF cantilever 326 is included in the device.As illustrated in FIG. 10B, the device includes a second lens 330 tofocus the beam after processing by the MEMS scanner 328. A lens support332 can also be included.

The many features and advantages of the invention are apparent from thedetailed specification, and thus, it is intended by the appended claimsto cover all such features and advantages of the invention which fallwithin the true spirit and scope of the invention. Further, sincenumerous modifications and variations will readily occur to thoseskilled in the art, it is not desired to limit the invention to theexact construction and operation illustrated and described, andaccordingly, all suitable modifications and equivalents may be resortedto, falling within the scope of the invention.

1. A device for obtaining OCT images from a subject comprising: aminiature OCT imaging probe configured to obtain high-resolution imagesof the subject; and a diffraction element configured to mitigatewavelength dependent aberration in the high-resolution images obtainedby the OCT imaging probe.
 2. The device of claim 1 wherein thediffraction element comprises a diffractive lens.
 3. The device of claim2 wherein the diffractive lens is positioned at a distal end of acompound lens within the OCT imaging probe.
 4. The device of claim 2wherein the diffractive lens comprises a high diffraction efficiencyover a broad spectral range.
 5. The device of claim 4 wherein the broadspectral range is between approximately 800 to approximately 1050 nm. 6.The device of claim 1 wherein the wavelength dependent aberrationcomprises a chromatic aberration.
 7. A method for mitigating achromaticaberration in OCT imaging comprising: using a diffraction elementintegrated into miniature imaging optics of an OCT imaging probe,wherein the OCT imaging probe comprises a broadband lightsource;reducing a longitudinal focal shift of the broadband light source, suchthat different colors of light in the broadband light source are focusedto a small spot for achieving high lateral resolution; and minimizingdistortion to a back reflected spectral at a given imaging depth, suchthat OCT axial resolution is improved to an optimal axial resolutionafforded by the broadband lightsource.
 8. A device for obtaining OCTimages from a subject comprising: a miniature OCT imaging probeconfigured to obtain high-resolution images of the subject; a PZTactuated fiber scanner; and a diffraction element configured to mitigatewavelength dependent aberration in the high-resolution images obtainedby the OCT imaging probe.
 9. The device of claim 8 wherein thediffraction element comprises a diffractive lens.
 10. The device ofclaim 9 wherein the diffractive lens is positioned at a distal end of acompound lens within the OCT imaging probe.
 11. The device of claim 9wherein the diffractive lens comprises a high diffraction efficiencyover a broad spectral range.
 12. The device of claim 11 wherein thebroad spectral range is between approximately 800 to approximately 1050nm.
 13. The device of claim 8 wherein the wavelength dependentaberration comprises a chromatic aberration.
 14. A device for obtainingOCT images from a subject comprising: a miniature OCT imaging probeconfigured to obtain high-resolution images of the subject; a MEMS beamscanner; and a diffraction element configured to mitigate wavelengthdependent aberration in the high-resolution images obtained by the OCTimaging probe.
 15. The device of claim 14 wherein the diffractionelement comprises a diffractive lens.
 16. The device of claim 15 whereinthe diffractive lens is positioned at a distal end of a compound lenswithin the OCT imaging probe.
 17. The device of claim 15 wherein thediffractive lens comprises a high diffraction efficiency over a broadspectral range.
 18. The device of claim 17 wherein the broad spectralrange is between approximately 800 to approximately 1050 nm.
 19. Thedevice of claim 14 wherein the wavelength dependent aberration comprisesa chromatic aberration.